There are already suggested such sensors of the theft and the like of an automobile as a high frequency vibration sensor wherein, in order to avoid a mis-operation caused by a foreign acoustic noise or the like, a resonance system having a resonance frequency above 2 KHz is formed to take out as a sensing signal the vibration of the car body near that resonance frequency and a low frequency vibration sensor wherein a resonance system having a resonance frequency below several 10 Hz is formed to take out as a sensing signal the vibration of the car body near that resonance frequency.
However, there has been a defect that the high frequency vibration above 2 KHz will attenuate so much in case it is transmitted through the car body that the place of fitting the above mentioned high frequency vibration sensor is limited. On the other hand, under the influence of the buffer springs and elastic rubber bodies used in the car body, the low frequency vibration below several 10 Hz will attenuate comparatively so little as to well propagate to the corners of the car body and therefore the place of fitting the above mentioned low frequency vibration sensor need not be selected.
Therefore, the above mentioned low frequency vibration sensor has been mostly used as a sensor of the theft and the like of an automobile.
However, in the already provided low frequency vibration sensor, the resonance system is formed, for example, of only a thin plate fixed at one end, therefore the resonance sharpness is so large that, in case the specific conditions are not met at the time of a theft or, for example, the kind of the automobile and the loaded weight are different, the theft and the like of the automobile will not be likely to be positively sensed.
Therefore, there has come to be provided a sensor wherein a resonance system having a resonance frequency above several 10 Hz is formed, a low pass filter of a cut-off frequency of several 10 Hz is provided and a feeble signal below several 10 Hz in the attenuating range of the above mentioed resonance system deviated from the above mentioned resonance frequency is amplified by an amplifier to be a sensing signal.
However, according to this sensor, there have been defects that, though all the vibrations of the car body below the frequency of several 10 Hz can be detected, the amplitude of the above mentioned amplifier will be practically required to be above 100 dB, the influence of the noise and the like generated from the amplifier and current source will not be negligible and the sensor will be costly. Further, in this sensor, there has been a defect that the signal is so feeble as described above that, in case a piezo-electric element is used for the signal detecting element, due to the quick ambient temperature fluctuation, the pyroelectricity produced in the piezo-electric element will not be negligible.
Further, in the above mentioned already provided sensor, as the resonance system is formed of only a mere thin plate, the vibration of the car body in the direction along the surface of the thin plate can not be sensed and, in this respect, too, the theft and the like of the automobile have not been likely to be positively sensed.